Well, there is a "nasty" but effective way to do that, although
please note I am against it unless all else has failed and you must do it.
It simply involves adding a dummy socket to one or more of the
fd_sets, and triggering it when you want the select call to return.
The ugly trigger is closing the socket. This could involve several
problems but has worked for me before (use the read_fds for this).
You're right, this is a kind of nasty thing.
A more subtle trigger is a read or write to the socket. You should
define a DATAGRAM socket, write a byte when you want select to return,
and don't forget to consume this byte (using recv) when select
actually returns.
I thought this was not possible with lwip.
I think this is the way to go.
But, right now, I have something really weird in my system since even
with only one thread (using timeout), send() or select() locks when data
are received one the cable while other data are transmitted by the
application.
Thanks for your help.
Nicolas
HTH,
Yoav.
On 11/01/10 16:24 PM, Nicolas Pinault wrote:
Yoav Nissim a écrit :
Simon,
Is the half duplex limitation only a socket implementation issue?
other than the missing locks, what else is missing? using the
netconn API seems to result in the same problem.
Isn't the TCPIP thread performing operations one at a time anyway?
Nicolas,
I'm sorry. I tried thinking about this for a while but the only way
I could produce any helpful code here is if I throw myself into
actually coding and testing it... my boss would not like that I
imagine :)
Ok. I understand. No problem.
Are you working with existing vnc server code that uses two threads,
or are you writing the code yourself?
My code is based on a code I have found in eCos svn tree. The
original code uses 2 threads. One for receiving commands from client
and one for sending screen content.
If you have existing code, maybe you should try serializing the send
and receive operations with a separate thread using select as I've
suggested earlier.
I have modified my code to use only one thread. The result is
catastrophic. It works worse than ever. When trying to transmit,
select() terminates with a timeout (1sec)
If you are writing your own code, could you clarify why a single
thread (not the main) using select and doing all Rx and Tx would not
work?
select() blocks until data are received or timeout occurs.
The system screen is updated by another thread. When the screen has
been updated, screen content must be sent to the client.
How do you go out of select() if you don't use timeout ?
Nicolas
Yoav.
On 11/01/10 11:33 AM, Nicolas Pinault wrote:
Simon,
Simon Goldschmidt a écrit :
2. Keep Rx and Tx threads locked and waiting on a select running in
another thread (in essence adding the locking mechanism that lwIP
does
not support)
The idea is good, but it's not what lwIP doesn't support, since
you don't get full duplex but mutual exclusion for read & write.
Full duplex would allow simultaneously sending and receiving on
the wire. However, that's the best you can get, currently, when
using sockets.
My problem is not to simultaneously receive and transmit data.
My problem is that I have an application that receives data and
transmit data asynchronously.
My application is a vnc server.
Vnc server receives data from client when, for example, the user
moves the mouse or hits keys.
Vnc server sends screen content to client when needed.
The client can request the server to force send screen content. In
this case, no problem.
But vnc server can also need to send screen content while nothing
is received.
Two threads are ideal in this situation. One for receiving commands
and one for sending screen content.
I can't see any other solution than using select with a short timeout.
Nicolas
Simon
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Yoav Nissim
Software Engineer, Software Tools Division
Jungo Software Technologies
Email: [email protected]
Web: http://www.jungo.com
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[email protected]
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--
Yoav Nissim
Software Engineer, Software Tools Division
Jungo Software Technologies
Email: [email protected]
Web: http://www.jungo.com
Phone: +972-74-7212138
Fax: +972-74-7212122
Mobile: +972-54-2271315
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